It has previously been known that a post may be placed in an endodontically treated tooth to retain a superstructure that replaces missing coronal tooth structure. Thus, the retention of the crown is greatly enhanced. Posts have been employed in refabrication for several centuries. Over the years, posts of varying configuration have been cemented with various cements and with varying degrees of success and acceptance. Presently the two most popular cements are zinc oxyphosphate cement, which has been used since 1900, and polycarboxylate cement introduced in 1968.
Placement of an artificial crown requires that the remaining structure of the tooth be properly prepared by instrumentation after the root canal space has been cleaned, shaped and filled. Generally, an interior space is machined into the root canal. Irrigation usually accompanies instrumentation and serves to cool and lubricate the cutting tool while flushing the removed material away from the machined site. Once the root canal is prepared, a post of appropriate size and shape is cemented into the canal. After the cement has sufficiently hardened, a mass around the top of the post is built-up of various substances, to form a core. Generally, preformed crowns are available and have a hollow area to accept such a build-up mass. These are placed as a temporary cover until a final crown has been made. This final crown is placed upon the prepared tooth structure and bonded to the tooth and core using various cements.
In the past few years, there has been considerable discussion about the inability of chemomechanical procedures to thoroughly remove all the cellular debris within the root canal space. Silicone models are used to demonstrate the great irregularity and complexity of the root canal space. They illustrate that even mechanically well-prepared canals harbored areas that were never contacted by endodontic instruments. Scanning electron microscope (SEM) studies of the effects of mechanical preparation revealed that, regardless of the technique used, often pulp tissue remained and areas of the canal walls were not instrumented. See Mizrahi et al., "A scanning electron microscopic study of the efficacy of various endodontic instruments", J. Endod.1(10): 324-33, (1975); Moodnick et al., "Efficacy of Biomechanical instrumentation: a scanning electron microscopic study," J. Endod. 2(9): 261-66, (1976); Bolanos and Jensen, "Scanning electron microscope comparisons of the efficacy of various methods of root canal preparation," J. Endod. 6(11): 815-22, (1980). In addition, other studies using the SEM found that many of the commonly-used irrigating solutions were also ineffective in completely removing hard and soft tissue debris, especially in the apical portion of the canal. See Baker et al., "Scanning electron microscopic study of the efficacy of various irrigating solutions," J. Endod. 1(4): 127-35, (1975); McComb and Smith, "A preliminary scanning electron miscroscopic study of root canals after endodontic procedures," J. Endod. 1(7): 238-42, (1975); Rubin et al., "The effect of istrumentation and flushing of freshly extracted teeth in endodontic therapy: a scanning electron miscroscopic study," J. Endod. 5(11): 328-35, (1979). These results show that the currently-accepted methods of chemomechanical preparation were inadequate in preparing a debris-free canal.
Thus, emphasis has been placed on improving the manufacture of endodontic instruments and developing more effective irrigation techniques and endodontic materials.
Since instrumentation is not entirely effective in cleaning the entire canal, the solutions used should help remove pulp tissue remnants, necrotic debris and bacteria remaining in the prepared root canal space without irritating the periapical tissue.
Investigators have also described a sludge or smeared layer that exists on portions of the canal walls. This appears as an amorphous layer on the canal wall that obstructs the dentinal tubules. It was recently demonstrated that the smeared layer is primarily calcific in nature and is created by instrumentation. See Goldman et al., "The efficacy of several Endodontic irrigation solutions: a scanning electron microscopic study, " Oral Surg. 52(2): 199-204, (1981).
The smeared layer blocks the dentinal tubules. A recent study has shown that this calcific layer reduced the permeability of dentin in vitro by more than forty percent. See Dippel et al., "Influence of the smeared layer and intermediary base materials on the permeability of dentin," J. Dent. Res. 60(B):1211, (1981).
Therefore, even well-instrumented canals could contain organic debris such as pulp tissue as well as inorganic debris such as a smeared layer. Recent investigations raised the question of removing both layers. See Wayman et al., supra and Koskinen et al., "Appearance of the chemically treated root canal walls in the scanning electron microscope," Scand. J. Dent. Res. 88(5):397, (1980). However, those attempts were unsuccessful.
Similarly, it has been found that the smeared layer is present in a cavity and prevents an optimum bonding of the filling placed in the cavity. This permits leakage and subsequent entry of food particles and bacteria, which cause further deterioration of the tooth structure.